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Abstract:

Permanent magnet assemblies include a central cylindrical magnet having a
bore. The cylindrical magnet is magnetized along a selected radial
direction and is enclosed within a ferromagnetic shim. A uniform magnetic
field, field gradient, or other field distribution can be produced in the
bore based on the bore cross-sectional shape.

Claims:

1. A magnet assembly, comprising: a permanent magnet defining an air gap;
a ferromagnetic shim situated about the permanent magnet.

2. The magnet assembly of claim 1, wherein the permanent magnet and the
ferromagnetic shim are cylindrical, the ferromagnetic shim is situated so
as to be concentric with the permanent magnet, and a permanent magnet
magnetization is directed so as to be in a plane perpendicular to an axis
of the permanent magnet.

3. The magnet assembly of claim 2, wherein the cylindrical permanent
magnet has a magnetization that is directed along a diameter of the
cylindrical cross section of the cylindrical permanent magnet.

4. The magnet assembly of claim 3, wherein the air gap in the permanent
magnet has a circular cross-section concentric with the permanent magnet.

5. The magnet assembly of claim 3, wherein the air gap in the permanent
magnet has a square cross-section centered with the permanent magnet and
having a diagonal of the square cross-section aligned with the
magnetization of the permanent magnet.

6. The magnet assembly of claim 1, further comprising a Halbach array
situated in the air gap in the permanent magnet.

7. A method, comprising: selecting a magnetic field distribution in at
least one plane; providing a ring permanent magnet having an internal air
gap associated with the selected magnetic field distribution, wherein the
magnetization is parallel to the plane; and situating a ferromagnetic
shim about the permanent magnet.

8. The method of claim 7, wherein the magnetization of the magnet is
parallel to a diameter of the ring.

9. The method of claim 7, wherein the internal air gap has a circular
cross-section.

10. The method of claim 7, wherein the ring magnet and the shim are
circular cylinders.

11. The method of claim 7, wherein the ring magnet and the shim have
non-circular cross-sections in a plane parallel to the magnetization.

12. The method of claim 7, wherein the internal air gap has a rectangular
cross-section.

13. The method of claim 12, wherein a diagonal of the rectangular
cross-section is aligned with the magnetization of the permanent magnet.

14. The method of claim 7, wherein the cross-section of the shape of the
air-gap is a circle or polygon.

15. The method of claim 7, wherein the cross-sections of the shape of the
inner and/or outer surfaces of the magnet are a circle or polygon.

16. The method of claim 7, wherein the cross-section of the shape of the
inner and/or outer surface of the shim is a circle or polygon.

17. The method of claim 7, wherein the air gap, the outer surface of the
magnet, and the inner and outer surfaces of the shim have similar shapes.

18. The method of claim 7, wherein the air gap, the outer surface of the
magnet, and the inner and outer surfaces of the shim have similar shapes
that are aligned with respect to each other.

19. A magnet assembly, comprising: a magnetized cylinder having a central
bore and having a magnetization that is along a direction of a selected
radius of the cylinder; a ferromagnetic shim situated about the
magnetized cylinder; and a first set of cylindrical magnetics and a
second set of cylindrical magnets alternately situated at an inner
surface of the central bore such that the first set of magnets have
magnetizations parallel to the magnetization of the magnetized cylinder
and the second set of magnets have magnetizations perpendicular to the
magnetization of the magnetized cylinder.

20. The magnet assembly of claim 19, wherein the ferromagnetic shim is
spaced apart from the magnetized cylinder so as to form an air gap.

21. The magnet assembly of claim 20, wherein the ferromagnetic shim
comprises first and second half cylindrical shells situated to form a
cylindrical shell about the magnetized cylinder and the magnetized
cylinder comprises a plurality of sections.

22. The magnet assembly of claim 19, wherein the magnets of the first set
of magnets have alternating magnetizations and the magnets of the second
set of magnets have alternating magnetizations.

Description:

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. Provisional Application
61/496,362, filed Jun. 13, 2011 which is incorporated herein by
reference.

FIELD

[0003] The disclosure pertains to ring magnets and applications thereof.

BACKGROUND

[0004] Magnetic field patterns are critical for some applications. For
example, a gradient magnetic field is required for magnetic separation,
whereas a highly uniform field is required for magnetic detection using
NMR. Magnetic fields for magnetic separation and detection are typically
obtained using an array of precisely oriented permanent magnets such as a
quadrupole magnet or a Halbach array. See, for example, Blumich et al.,
U.S. Patent Application Publication 2010/0013473, which is incorporated
herein by reference. Precise alignments of the multiple magnets required
by these configurations can make the fabrication of such magnetic
circuits difficult, time consuming and expensive. In addition, tedious
user re-alignments can be required.

SUMMARY

[0005] According to representative examples, magnet assemblies comprise a
permanent magnet defining an air gap and a ferromagnetic shim situated
about the permanent magnet. Typically, the permanent magnet and the
ferromagnetic shim are cylindrical, and the ferromagnetic shim is
situated so as to be concentric with the permanent magnet. A permanent
magnet magnetization is directed so as to be in a plane perpendicular to
an axis of the permanent magnet. In some examples, the cylindrical
permanent magnet has a magnetization that is directed along a diameter of
the cylindrical cross section of the cylindrical permanent magnet. In
other examples, the air gap in the permanent magnet has a circular
cross-section concentric with the permanent magnet. According to other
examples, the air gap in the permanent magnet has a square cross-section
centered with the permanent magnet and a diagonal of the square
cross-section is aligned with the magnetization of the permanent magnet.
In still further examples, a Halbach array is situated in the air gap in
the permanent magnet.

[0006] Methods comprise selecting a magnetic field distribution in at
least one plane and providing a ring permanent magnet having an internal
air gap associated with the selected magnetic field distribution, wherein
the magnetization is parallel to the plane. A ferromagnetic shim is then
situated about the permanent magnet. In some examples, the magnetization
of the magnet is parallel to a diameter of the ring and the internal air
gap has a circular cross-section. In other representative embodiments,
the ring magnet and the shim are circular cylinders. In some embodiments,
the ring magnet and the shim have non-circular cross-sections in a plane
parallel to the magnetization. In some embodiments, the internal air gap
has a rectangular cross-section. In other examples, a diagonal of the
rectangular cross-section is aligned with the magnetization of the
permanent magnet. In other embodiments, the cross-section of the shape of
the air-gap is a circle or polygon. In some examples, the cross-sections
of the shape of the inner and/or outer surfaces of the magnet are a
circle or polygon. In still further examples, the cross-section of the
shape of the inner and/or outer surface of the shim is a circle or
polygon. In further examples, the air gap, the outer surface of the
magnet, and the inner and outer surfaces of the shim have similar shapes
that are aligned with respect to each other.

[0007] Magnet assemblies comprise a magnetized cylinder having a central
bore and having a magnetization that is along a direction of a selected
radius of the cylinder. A ferromagnetic shim is situated about the
magnetized cylinder. A first set of cylindrical magnets and a second set
of cylindrical magnets are alternately situated at an inner surface of
the central bore such that the first set of magnets have magnetizations
parallel to the magnetization of the magnetized cylinder and the second
set of magnets have magnetizations perpendicular to the magnetization of
the magnetized cylinder. In some examples, the ferromagnetic shim is
spaced apart from the magnetized cylinder so as to form an air gap. In
other examples, the ferromagnetic shim comprises first and second half
cylindrical shells situated to form a cylindrical shell about the
magnetized cylinder and the magnetized cylinder comprises a plurality of
sections.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1A is a perspective view of a magnet assembly that includes a
cylindrical magnet having a central bore situated inside of and coaxial
with a ferromagnetic cylindrical shim

[0009] FIG. 1B is a sectional view of a magnet assembly such as that of
FIG. 1A illustrating a diametrically magnetized ring magnet situated
within a circular bore of a coaxial ferromagnetic cylindrical ring shim
defining an air gap in which selected magnetic field distributions can be
produced.

[0010] FIG. 1C is a sectional view of a magnet assembly such as the magnet
assembly of FIG. 1B.

[0011] FIGS. 2A-2C illustrate modeling results showing field distributions
for a shimmed ring magnet design with circular air gap in which
IDM=1 cm, ODM=3.2 cm, IDS=3.2 cm, ODS=7.6 cm, and
SMS=0.0 cm. FIG. 2A illustrates the ring magnet assembly, wherein
arrows indicate the direction of magnetic field inside the magnet. FIG.
2B is a shaded representation showing a uniform magnetic field
distribution in the air gap. FIG. 2C is plot of calculated magnetic flux
density along the X and Y axes inside the air gap.

[0013] FIG. 3B is a plot showing improvement in magnetic flux density with
increasing magnet thickness until a certain threshold is reached.
Variable magnetic fields ranging from 0.2 to 0.55 T can be produced for
this configuration.

[0014] FIG. 4A illustrates a shimmed ring magnet and associated magnetic
fields with a varying gap spacing (SMS) ranging between 0 cm and
12.7 cm. The thickness of the magnet and the air gap diameter were fixed
at 3.8 cm and 1 cm, respectively.

[0015] FIG. 4B is a plot showing variable magnetic flux densities between
0 to 0.55 T inside the air gap with changing spacing between the magnet
and the iron core.

[0016]FIG. 5 illustrates a shimmed ring magnet combined with a Halbach
magnet arrangement and the associated magnetic fields. In this example,
the Halbach arrangement includes eight circular magnets (diameter=2.5 mm)
that are arranged with alternating magnetizations magnetized inside the
central air gap. In this example, IDM=1 cm, ODM=3.8 cm,
SMS=0.0 cm, IDS=3.8 cm and ODS=7.6 cm. Magnetic field
directions inside the magnets are indicated by arrows. High magnetic
fields (˜1.1 T) were calculated for this configuration.

[0017] FIGS. 6A-6C illustrate modeling results showing field distributions
in a shimmed ring magnet having a square air gap. FIG. 6A shows a shimmed
ring magnet assembly having a 3.2 cm square opening (ODM=7.6 cm,
SMS=1.0 cm, IDS=7.6 cm and ODS=12.7 cm), wherein arrows
indicate the field direction inside the magnet. FIG. 6B illustrates
gradient magnetic field distribution in the air gap and FIG. 6C is a plot
of calculated magnetic flux density as a function of position along the X
and Y directions inside the square air gap.

[0018] FIGS. 7A-7B illustrate various magnet configurations.

[0019]FIG. 8 is a sectional view of a shimmed ring magnet that is formed
of a ring magnet and a shim provided as sections.

[0020] FIGS. 9A-9B illustrate calculated magnetic fields produced with a
magnet assembly with an air gap having a circular cross-section defined
in a rectangular magnet surrounded by a rectangular air gap with no
additional air gap between the magnet and the shim.

[0021] FIG. 9C is a plan view of the magnet assembly used in the
calculations of FIGS. 9A-9B.

[0022]FIG. 9D is a plot of magnetic flux density as a function of
position along both the x-axis and y-axis in the air gap of the magnet
assembly of FIG. 9C.

[0023] FIGS. 10A-10B illustrate calculated magnetic fields produced with a
magnet assembly with an air gap having a square cross-section defined in
a rectangular magnet surrounded by a rectangular air gap with no
additional air gap between the magnet and the shim.

[0024] FIG. 10C is a plan view of the magnet assembly used in the
calculations associated with FIGS. 10A-10B. As shown in FIG. 10C,
diagonals of the shim, the magnet, and the air gap are aligned with the
magnetization of the magnet.

[0025] FIG. 10D is a plot of magnetic flux density as a function of
position along both the x-axis and y-axis in the air gap in the magnet
assembly of FIG. 10C.

DETAILED DESCRIPTION

[0026] As used in this application, the singular forms "a," "an," and
"the" include the plural forms unless the context clearly dictates
otherwise. Additionally, the term "includes" means "comprises." Further,
the term "coupled" does not exclude the presence of intermediate elements
between the coupled items.

[0027] The systems, apparatus, and methods described herein should not be
construed as limiting in any way. Instead, the present disclosure is
directed toward all novel and non-obvious features and aspects of the
various disclosed embodiments, alone and in various combinations and
sub-combinations with one another. The disclosed systems, methods, and
apparatus are not limited to any specific aspect or feature or
combinations thereof, nor do the disclosed systems, methods, and
apparatus require that any one or more specific advantages be present or
problems be solved. Any theories of operation are to facilitate
explanation, but the disclosed systems, methods, and apparatus are not
limited to such theories of operation.

[0028] Disclosed herein are ring magnet assemblies that include ring
magnets and co-axial ferromagnetic shims that can be configured to
produce magnetic field patterns suitable for magnetic separation and
detection applications. In some examples, substantially uniform magnetic
fields or gradient magnetic fields can be produced. Several magnet
configurations have been evaluated and/or optimized using commercial
finite element modeling software, and magnetic field distributions for
several representative configurations are provided herein. The disclosed
designs typically include a single ring magnet with a co-axial shim ring
so that alignment can be simple and straightforward. In representative
examples, the disclosed magnets comprise a circular cylinder permanent
magnet and a circular cylinder ferromagnetic shim. In some examples, the
lengths of the cylindrical magnet and the ferromagnetic shim are less
than an outer diameter of the permanent magnet, and are ring-like in
appearance. For convenient explanation, such cylindrical parts (with or
without bores) are referred to herein in some places as rings. In some
examples, particularly for large magnets, rings or cylinders can be
formed of multiple pieces for ease of fabrication. As shown below,
ferromagnetic shims are configured to be situated exterior to the ring
magnet, and in some examples, can slide along the outside surface of the
ring magnet.

[0029] With reference to FIG. 1A, a shimmed magnet assembly 100 includes a
ring (cylindrical) magnet 104 situated along an axis 101 and defining an
inner air gap 102. A shim ring (shim cylinder) 108 is situated along the
axis 101 about the ring magnet 104 so that a shim space 106 is defined
between the ring magnet 104 and the shim ring 108. Typically, the ring
magnet 104 and the shim ring 108 have circular cross sections in an
xy-plane and are centered on the axis 101. The magnet 104 and the shim
108 are generally selected to have lengths sufficient to reduce edge
effects in the magnetic field produced in the air gap 102. The ring
magnet 104 is magnetized to have a uniform field along a direction in the
xy-plane. For convenience, this direction can be assumed to be an
x-direction. The shim 108 can be made of any ferromagnetic material as
convenient.

[0030] FIG. 1B illustrates a cross-section of a shim ring magnet 150 such
as that of FIG. 1A. This magnet design comprises a ring shaped permanent
magnet 152 of internal diameter IDM, outer diameter ODM, and
magnet depth DM measured along the z-axis surrounded by a co-axial
ferromagnetic shim 154 of internal diameter IDS, outer diameter
ODS, and shim depth DS along the z-axis. The ring magnet 152
and the shim 154 may define an annular shim/magnet air gap of spacing
SMS. The ring magnet 152 is magnetized along a diameter so as to be
directed within the xy-plane and perpendicular to the z-axis. Any desired
magnetic fields (such as uniform or gradient magnetic fields) can be
produced in an air gap 160. Field distributions can be selected based on
a cross-sectional shape of the air gap. For example, a circular gap
provides a uniform magnetic field, while a polygonal (e.g., rectangular)
gap provides a gradient magnetic field.

[0031] FIG. 1C is an elevational sectional view of a magnet assembly such
as that of FIG. 1B. As shown in FIG. 1C, a permanent magnet 124 defines
an aperture 122 that extends along an axis 130. A shim 128 is situated
about the magnet 124 so as to define an air space 126. In some examples,
the air space 126 is omitted and the shim 128 contacts the magnet 124. As
shown in FIG. 1C, the depth of the magnet 124 (DM) is less than the
depth of the shim 128 (DS) so that the magnet 124 can be situated
within the shim 128. However, in other examples, the magnet 124 has a
greater thickness than the shim 128 and extends beyond the shim 128.
Regardless of depths, the shim 128 and the magnet 124 can be situated so
that the magnet extends out of the shim at at least one end. In addition,
the magnet 124 need not be placed symmetrically along the axis 130 with
respect to the shim 124 with respect to any of the x, y, or z-axes.

[0032] In the examples of FIGS. 1A-1C, a variety of magnets and shim
materials can be used such as, for example, NdFeB and Fe, respectively.
The air gap of the examples of FIGS. 1A-1B can be configured to produce
selected field distributions. Substantially uniform magnetic fields can
be obtained with a circular air gap, and in one example, field uniformity
was superior to that of an equivalent Halbach ring.

[0033] Field results for an example magnet assembly that produces uniform
magnetic field are shown in FIGS. 2A-2C. Simulation shows a uniform
magnetic field inside the air gap for an NdFeB ring magnet (IDM=1
cm, ODM=3.2 cm) enclosed by an iron shim (IDS=3.2 cm and
ODS=7.6 cm). A uniform magnetic flux density inside the air gap can
be controlled by varying different geometrical aspects of the design. For
example: [0034] 1) Varying the ratio
(ODM-IDM)/(ODS-IDS). FIGS. 3A-3B show magnetic field
results for an example magnet assembly having calculated magnetic flux
densities ranging from 0.2 to 0.55 T as a function of
(ODM-IDM). [0035] 2) Varying the spacing between the magnet and
the shim (SMS). FIGS. 4A-4B show an example of calculated magnetic
flux density inside an air gap varying between 0 T and 0.55 T as a
function of SMS ranging between 0.0 cm and 12.7 cm. [0036] 3) The
designs disclosed can be used in conjunction with Halbach arrangements,
i.e. by placing a Halbach arrangement of magnets inside an air gap. For
example, by inserting a Halbach arrangement inside the air gap as shown
in FIG. 5, uniform magnetic flux densities of ˜1.1 T can be
obtained. [0037] 4) High gradient systems can also be generated by
inserting ferromagnetic structures such as steel wool, steel mesh etc.
inside the air gap of the disclosed magnet assemblies.

[0038] FIGS. 6A-6B pertain to a shimmed ring magnet for production of a
magnetic field gradient based on an air gap having a square
cross-section, and FIG. 7B illustrates shimmed ring magnets based on an
air gap having a circular cross-section for production of uniform fields.
FIG. 7A illustrates a Halbach configuration that can be used in an air
gap of a shimmed ring magnet assembly.

[0040] In further examples, the central air gap can be defined by a
regular or irregular polygon, or can be elliptical, arcuate, a
combination of a polygon and a curve such as a portion of a circle or
oval. The outside surface of the ring magnet can also assume these other
shapes, as desired. An air gap can be provided between a ring magnet and
the shim, or the shim can fit with substantially no air gap, or can have
an arbitrary shape. While typically the air gap internal to the ring
magnet is a central air gap, in other examples the air gap need not be
centered on an axis of the ring magnet or the shim, and the ring and the
shim can be arranged to be non-coaxial as well.

[0041] With reference to FIG. 8, a magnet assembly includes a magnet 804
comprising sections 804A-804D situated about an air gap 802, and a shim
808 comprising sections 8008A-808B, but magnets and shims can be produced
as different arrangements of segments. As shown in FIG. 8, the sections
804A-804D are magnetized so as to correspond to a diametrical
magnetization as assembled. In the example of FIG. 8, a gap 806 between
the ring magnet 804 and the shim 808 can be an air gap, or a non-magnetic
spacer can be provided, conveniently as a cylindrical shell of a suitable
material. In some examples, the air gap can be configured to accommodate
a specimen container. For example, a cross section of the air gap 802 can
be selected to be substantially the same as that of a specimen tube, or a
cylindrical shell of non-magnetic material can be situated in the air gap
802 having a bore sized to accommodate a specimen tube or other container
or specimen shape.

[0042] The examples above are based on concentric ring magnets and shims
for convenient explanation. In other embodiment, magnets and shims can be
provided in other shapes. For example, a magnet assembly can comprise a
co-axial rectangular magnet and a rectangular shim. Other examples
include triangular magnets and triangular shims, or arbitrary polygonal
magnets and corresponding polygonal shims. Typically, a magnet and a shim
are aligned coaxially, and the magnetization is orthogonal to the axis.

[0043] FIGS. 9A-9B illustrate calculated magnetic fields produced with an
air gap having a circular cross-section defined in a rectangular magnet
surrounded by a rectangular air gap with no additional air gap between
the magnet and the shim. A plan view of the magnet assembly is shown in
FIG. 9C, and FIG. 9D is a plot of magnetic flux density as a function of
position along both the x-axis and y-axis in the air gap. In the example
of FIGS. 9A-9D, a substantially constant flux density is produced in the
air gap. The magnet and the shim are assumed (for purposes of
calculation) to extend arbitrarily along the z-axis so that end effects
can be disregarded.

[0044] FIGS. 10A-10B illustrate calculated magnetic fields produced with
an air gap having a square cross-section defined in a rectangular magnet
surrounded by a rectangular air gap with no additional air gap between
the magnet and the shim. Diagonals of the shim, the magnet, and the air
gap are aligned with the magnetization of the magnet. A plan view of the
magnet assembly is shown in FIG. 10C, and FIG. 10D is a plot of magnetic
flux density as a function of position along both the x-axis and y-axis
in the air gap. In the example of FIGS. 10A-10D, a substantially constant
gradient flux density is produced in the air gap. The magnet and the shim
are assumed (for purposes of calculation) to extend arbitrarily along the
z-axis so that end effects can be disregarded.

[0045] Magnets can be formed of any of a variety of materials such as are
known, including, for example, FeNdB and SmCo materials. Shims can
similarly be formed of any of a variety of ferromagnetic materials as
desired.

[0046] The examples described above are provide for convenient
illustration and are not to be taken as limiting the scope of the
disclosure. We claim all that is encompassed by the appended claims.